The old heating systems have made way for cleaner-burning, wood-fuelled boilers at 31 schools where students' chilly fingers will be warmed with heat from small pellets of dust and shavings left over from saw mills.

Between them the converted boilers are expected to produce 3300 fewer tonnes of CO2 a year than their coal-fired predecessors - the equivalent of taking more than 1000 cars off the road.

Simon Peek, deputy principal of Macleans College in Bucklands Beach, said the old school boiler had been given a new lease of life.

His school is the first in Auckland to convert to wood pellets.

"As well as a cleaner fuel we wanted a low-carbon, renewable option and we liked the idea that we didn't have to buy a new gas boiler or a large number of heat pumps," he said.

The Government-funded Energy Efficiency and Conservation Authority provided grants towards converting boilers at the 31 schools and will offer funding to more schools if the pilot goes well.

Pine pellets - which cannot be burned efficiently in an ordinary fire-place - are shovelled in and sparked into action by an electric element.

The combustion chamber is designed to produce only minimal smog compared with coal. The pellets are popular in Europe but just starting to catch on in New Zealand, where five plants produce them.

Minimal ash means the new burners require less frequent cleaning and the wood ash can be used as a fertiliser supplement on school gardens.